Not for the .wtp files. While they look like plain .html files they aren't. They contain WW tags that only WW expands.
So you open the .wtp file in your favorite text editor, I like PFE and PSPad but Notepad works, and you open the .html file that it creates into your favorite browser, and you open the WW forecast window. You then make your changes to the .wtp file, save it, you click on the Update button in the WW forecast window, then the Refresh button on your browser and viola you see the results of your changes.
If your desired changes revolve strickly around the HTML code you can edit the .html files directly with a WYSIWYG editor then copy your HTML changes to the .wtp file. With this approach you need to either rename the WW .html file you are editing or disable WW's auto updates.
If anyone uses a different approach it would be nice to hear about it.
A testing feature I would like to have is a way to direct WW.exe to call a version of DL.exe that would read a text file of test weather data rather than the live weather.com data.